In the reproduction of continuous tone information for mechanical printing purposes, it is customary to make a half-tone photographic intermediate, usually a film negative, in which the gradations in tone are represented by dots of differing size. The quality of the resulting halftone picture is closely connected with the shape, spectral density, and uniformity of the dots of the half-tone print.
In order to obtain very high-contrast screen dots in halftone images it has been practice to formulate developers, so-called lith-developers, containing essentially a p-dihydroxybenzene such as hydroquinone, an alkali, an alkali metal bromide and a low level of free sulphite ions.
Very high contrast results, preferably with gamma above 10, also called "lith-gradation", can be obtained with said high-contrast developers and so-called "lith silver halide emulsion materials". In these materials the silver halide comprises at least 50 mole % of chloride, the balance, if any, being bromide and optionally a minor amount of iodide.
Hydroquinone developers having a low sulphite ion concentration are commonly referred to as "lith-type developers" and their mechanism of operation is described by J. A. C. Yule in the Journal of the Franklin Institute, 239 (1945), pages 221 to 230.
In conventional "lith" developers the low sulfite ion concentration is inadequate to provide effective protection against aerial oxidation. As a result, a conventional "lith" developer is lacking in stability and tends to give erratic results depending on the length of time that it has been exposed to air.
As explained e.g. in U.S. Pat. No. 4,081,280 it is necessary when using a lith-developer with low free sulphite content to replenish carefully the developer solution compensating:
(1) for developer exhaustion by aerial oxidation, and PA1 (2) for use of developer in function of the treated exposed photographic material. PA1 a) PA1 b) free sulphite ions in an amount of at least 5 g per liter, and PA1 c) an organic anti-fogging agent corresponding to one of the following general formulae (A) and (B): ##STR2## wherein: Y.sup.1 is a nitro-substituent in the 5- or 6-position of the indazole nucleus, PA1 X.sup.1 is hydrogen or a sulphonic acid group in salt form, PA1 R is hydrogen or a lower (C.sub.1 -C.sub.5) alkyl group, PA1 X is a substituent being or containing a carboxyl group or sulphonic acid group in salt form, PA1 d) a polymer containing a plurality of alkylene oxide units and having a molecular weight of at least 1500, and said ingredients a) to d) being present in the aqueous developing medium in such relative amounts that development therein of the exposed photographic material carrying a latent continuous tone wedge image under the same development conditions given the density versus log exposure sensitometric curve of the resulting silver wedge would have a) a maximum gradient (.gamma.) of at least 5.0 between the log exposure values measured at densities of 0.3 and 3.0 above fog on the log exposure scale and b) a gradient (.gamma..sub.v) of at least 2.0 in the toe between the log exposure values measured at densities of 0.1 and 0.6 above fog on the log exposure scale. PA1 1) hydroquinone or a substituted hydroquinone as sole developing agent or said hydroquinone as main developing agent in combination with an auxiliary developing agent in an amount less than 10 mole % with respect to the main developing agent, PA1 2) an inorganic compound yielding free sulphite ions in an amount of at least 5 grams per liter, PA1 3) an organic anti-fogging agent corresponding to the following general formula (Z): ##STR3## wherein: X.sup.2 represents an alkyl group or an aryl group including said groups in substituted form or an amino group including said group in substituted form; PA1 the amount of anti-fogging agent being in the range of 50 mg to 1 g per liter, PA1 4) a polymer containing a plurality of alkylene oxide units, and PA1 5) the necessary alkaline material to have in said liquid a pH of at least 10.5, preferably between 10.8 and 11.8. PA1 1) hydroquinone or a substituted hydroquinone as sole developing agent or said hydroquinone as main developing agent in combination with an auxiliary developing agent in an amount less than 10 mole % with respect to the main developing agent, PA1 2) an inorganic sulfite compound providing free sulfite ions in an amount of at least 5 grams per liter, PA1 3) an organic anti-fogging agent corresponding to the following general formula (Z): ##STR4## wherein: X.sup.2 represents an alkyl group or an aryl group including said groups in substituted form or an amino group including said group in substituted form, and PA1 4) a polymer containing a plurality of alkylene oxide units and having a molecular weight of at least 1500, said ingredients 1), 2), 3) and 4) being present during said development in said aqueous medium in such amounts that when said photographic material is image-wise exposed through a continuous tone wedge and test developed in the thus constituted aqueous developing medium for a time of 45 s at 30.degree. C. and fixed there results a silver wedge image corresponding with a log exposure versus density sensitometric curve wherein the maximum gradient (.gamma.) is at least 5 and the product of said maximum gradient (.gamma.) and of the gradient in the toe (.gamma.v) of said curve is at least 20; the maximum gradient is measured between the log exposure values corresponding with the densities 0.3 and 3.0 above fog of said curve and the gradient in the toe is measured between the log exposure values corresponding with the densities 0.1 and 0.6 above fog of said curve.
According to U.S. Pat. No. 3,972,719 in order to be less dependent on replenishment for aerial oxidation a high contrast developer with relatively high sulphite content and an anti-fogging nitro-compound is provided. Said developer contains not more than 0.05 g/liter of any auxiliary developing agent that shows a superadditive developing effect with a p-dihydroxybenzene developing agent and may contain a polymer containing a plurality of alkylene oxide units, i.e. a polymeric oxyalkylene compound, for controlling the development speed.
According to published European Patent Application 0 196 705 a method of effecting high contrast development of an imagewise exposed photographic silver halide emulsion layer material is carried out in an aqueous medium having a pH of 10 to 12 containing:
(i) hydroquinone or a substituted hydroquinone, PA2 (ii) an auxiliary developing agent having a developing activity of such degree as to give when utilized in the Standard Development Test defined in the specification a relative development rate f.sub.x in the range of about 1-2,
Until recently the characteristic of lith-development resulting in a particularly high gradation necessary for sharp screen dot reproduction was restricted to the use of silver halide emulsion layers the silver halide of which was mainly silver chloride.
According to U.S. Pat. No. 4,710,451 a method of effecting high contrast development of an image-wise exposed photographic silver halide emulsion layer material is provided, wherein an image-wise exposed silver halide emulsion material the silver halide of which is at least 90 mole percent silver bromide, the remainder if any, being chloride and/or iodide is developed in a developing medium containing hydroquinone as sole developing agent, a large amount of sulphite (at least 5 g per liter of free sulphite ions), a nitro-indazole as anti-fogging agent, a polyoxyethylene polymer having a molecular weight of at least 1500 and an inorganic alkaline compound to impart to the developing medium a pH of at least 10.5.
Since the nitro-indazole and other indazoles such as cyano- and halo-indazoles, particularly 5-cyano and 5-chloro-indazole, are instable in alkaline medium and loose their anti-fogging activity and development retarding activity, the developer formulation is kept in concentrated form in two parts before use an combined and diluted to the desired strength with water, the nitro-indazole antifogging agent(s) and polyoxyalkylene polymer being kept before use in acid medium in one part and the other ingredients in alkaline medium in the other part. Although by said procedure the stability problem is solved it would be more convenient if one could utilize only a single liquid high contrast developer that does not suffer from degradation on storage and use and avoid the need for separate media that have to be combined before use.